In a conventional railway coupler arrangement, the coupler head is connected through a shank to a yoke having a yoke strap which envelops a draft gear (i.e., a spring arrangement which absorbs shock loads). The draft gear, in turn, is carried within a sill which is secured to the car body and is provided with front and rear draft lugs between which the draft gear can slide. During draft loads (i.e., pulling loads), the yoke strap forces the draft gear against the front draft lugs and compresses the draft gear. During buff loads (i.e., pushing loads), the yoke pushes the draft gear against the rear draft lugs, again compressing it, but in the opposite sense.
In certain cases, it is desirable to provide a rotary coupler which permits an entire hopper car, for example, to be rotated and turned upside-down to discharge its contents without decoupling it from forward and aft cars. Many types of rotary couplers have been devised as exemplified by U.S. Pat. Nos. 3,104,017 and 4,090,614. In many of these designs, problems have been encountered because of the limited area available for the rotary connection between the front draft lugs. Other designs utilize the space at the coupler head to obtain adequate room for the rotary connection. In either case, design constraints either limit the diameter of the rotary shank or butt area (which are critical factors), or involve other drawbacks.